Hello Kevin,
You already have some great ideas. I'm adding somethings to keep more attention or you have concentrate on this things.
First thing is, the set design and light effects.
The first thing the audience will see is the set. If there is nothing to attract there eyes they will gonna lose attention. And the special light effects will help you to control there mind. Don't give them any chance to lose there attention.Second, the sound effects. Use something new and good to hear. You can use Full bass beats, or lead guitar or storm sound or else which suits your script and the story. But a great starting can grab the full attention of you audience.
And the last thing is voice over. I mean the script. Use easy words. Don't make complex. But keep the suspance.
That's all I can say.
My English is not good, if you can understand, please ask.